Re: Why is the push to such high step rates (Grex and Arc100)?

Hi Fred,

While I agree you might see an edm machine with a higher resolution
than .0001", it isn't common nor practical. As you know being a
machinist, dealing with anything smaller than .0001" requires very
stable temperature controls and even foundation isolation. While I
have been around EDM machines in about 10 years, back then they were
programmed to .0001" as the least input increment. The resolution
of the encoders should be finer than that, but again, with EDM you
do not have velocity so your SPS is still VERY LOW, much lower than
a metal cutting machine.

As for threading, yes, you do want good resolution there as well as
high speed, but even the commercial CNC lathes have limits as to how
fast you can thread to keep things in balance. Furthermore, the
threading precision on a CNC lathe is more closly tied to the
precision of the lead screw. If there isn't pitch error comp. in the
control, then no matter how good that encoder is, your still going
to end up with pitch error if the lead screw is bad.

I believe that on the MACH controlled lathes, they are getting good
threads even with a single "index" counter on the spindle without
any need at all for an encoder on the spindle.

But, like you said, very few hobby CNC machines challenge even
125,000 SPS.

Ah, I am obsessing about it I guess. I guess a practical example
can't be given for whatever reason so I should probably drop the
topic so as to not offend everyone.

Chris



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Fred Smith" <imserv@...>
wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "lcdpublishing"
> <lcdpublishing@> wrote:
> >
> > So, once again, I am back where I started, is there a practical
> > application?
>
> Wire EDM and grinding are programmed in the 10 millionths of

inches.

> In order to attain process capability, machine resolution with at

least

> an order of magnetitude smaller than these divisions is needed.

That

> yields .000001 inch resolution ( one millionth ).
>
> Threading is a good example of an application that benefits from

high

> speed and high resolution. Up to the max rpm of inexpensive

encoders,

> using a quadrature encoder for threading, improves the lead

accuracy as

> well as the range of available spindle rpms at which a given

thread-

> diameter combination can be machined. If you electronically gear

the

> spindle to an axis, the step rate of the axis is multiplied by the
> available spindle speed and will quickly out strip a stepper.

putting

> a hi resolution encoder on the axis motor will provide a good

balance

> to the spindle encoder.
>
> As you calculated, up to around 125000-135000 sps is about as fast

as

> is practical for hobby use. DeskCNC has been available at 125000

for a

> couple of years. Very few hobby CNC applications challenge that

rate.

>
> Fred Smith - IMService
> htp://www.imsrv.com
>